It's interesting. Some other users have complained that Firefox makes its window too tall and blocks the Task Bar.

Sometimes the file that stores window dimensions becomes corrupted. You can rename the file and have Firefox save settings from scratch, but this file also stores your toolbar customizations, so if this doesn't help, and you are facing a lot of having to move icons around, you might want to undo the change.

Open your current Firefox settings (AKA Firefox profile) folder using

Help > Troubleshooting Information > "Show Folder" button

Leave that window open, switch back to Firefox, and Exit (preferably using the menu)

Pause while Firefox finishes its cleanup, then rename localstore.rdf to something like localstore.old

Restart Firefox. Any improvement?

If you need to undo the change, you'll exit Firefox, delete the freshly updated localstore.rdf, then rename localstore.old to localstore.rdf.

Thanks for the suggestion. No change except for a few moments to find the menu which is normally just an icon. ;)

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Starting Fx in safe mode opens a small (probably VGA) window and restarts this way regardless of how you stretched it before closing.

I'm thinking it's my setup. I use classic windows settings. Also set fat coloured windows borders so I don't have to muck around to grab them. I also change defaults on just about everything display-wise. I run into incompatibilities on lots of software. This may be a bug but is pretty minor.